Many types of measurement instruments rely upon experimentally determined calibration curves to convert the raw data which is read by the instrument into an accurate measurement reading. Typically, the calibration curve is derived by taking measurement readings with the instrument on several samples whose composition has been determined analytically, and then constructing a calibration curve which relates the experimentally determined measurement readings to the analytically determined composition values. Because of minor variations from one measurement instrument to another, a calibration curve is unique for a particular instrument, and it is therefore necessary for each measurement instrument to be calibrated individually. The present invention provides a method and system which greatly facilitates the calibration procedure. This invention is described herein in terms of the calibration of a neutron gauge designed for measuring the asphalt content of bituminous paving mixes. This invention can, however, be embodied in many different forms and can be used with other types and designs of instruments which employ experimentally derived calibration curves.
Lowery, et al. U.S. Pat. No. 3,492,475 discloses a portable nuclear gauge which utilizes a fast neutron source and a thermal neutron detector for determining the composition of a bulk material, such as a bituminous paving mix, placed in a sample pan. This type of gauge relies upon the neutron moderating characteristics of hydrogen atoms present in the composition for determining, for example, the amount of asphalt in a paving mix or the amount of moisture in a building material. For these determinations it is known that the amount of asphalt or the amount of moisture can be related to the hydrogen content of the material, and the hydrogen content of the material can be determined by subjecting the sample to radiation from a fast neutron source and detecting neutrons which have been slowed or thermalized as a result of interaction with the hydrogen nuclei present in the sample. The number of thermalized neutrons detected (counted) over a period of time is utilized in determining the hydrogen content of the sample.
In operating the gauge, it is first necessary to establish a standard count for calibration purposes. This is accomplished using a standard sample having a known hydrogen content, for example, a block of polyethylene. Then calibration curves are produced for the particular material being tested, by using carefully prepared samples having a known content of the hydrogen-containing material of interest (e.g. asphalt or moisture). After the calibration curves have been produced, unknown test samples can be placed in the gauge and counts are taken. By reference to the calibration curve, the corresponding content of the hydrogen-containing material for that count can be read.
A more recent model of this gauge has been produced by applicant's assignee embodying the principles of the Lowery patent and sold as the "Model 3241 Asphalt Content Gauge" by Troxler Electronic Laboratories, Inc. This gauge includes a microprocessor to facilitate calibration and computation of the sample asphalt content. Calibration can be made by taking gauge counts on two or more samples of known asphalt content. The microprocessor then constructs a calibration equation from these data points, and the gauge provides a direct readout of the percent asphalt, thus eliminating the necessity of calculations and reference to external calibration tables.
In order to obtain the most accurate measurements, the gauge must be calibrated each time the composition of the material is changed. This is because the number of counts recorded is only representative of the hydrogen atoms present in the sample. There is an assumption made when using a thermal neutron gauge that the differences in hydrogen count from sample to sample are because of changes in the amount of the substance of interest, such as moisture or asphalt content, and that all other factors are maintained substantially constant. The calibration is done when it is clear that the "other factors" are not going to be constant. Such changes may occur, for example, when using a new aggregate in the paving mix or a new source or grade of asphalt. A new aggregate may have a different average moisture content or a different intrinsic hydrogen content. In the case of asphalt, different sources of asphalt may have a different concentration of hydrogen. At a time when there is such a change, the gauge must be calibrated using carefully prepared samples of known concentrations of the hydrogen-containing material of interest.
As discussed above, the calibration procedure involves taking hydrogen counts with the gauge using several samples of known composition, and establishing a correlation, (e.g. an equation or a calibration curve) which can be used to obtain a percent asphalt reading from the hydrogen counts obtained from a test sample of unknown composition. The calibration procedure itself is not unduly complex, and is practical with a single gauge or where a relatively few gauges are involved. However, where a number of field gauges are used, as is frequently the case in many operations, the necessity of manually calibrating all the gauges becomes quite burdensome and time consuming. The gauges generally need to be taken out of the field and sent to a lab where samples of the new aggregate can be carefully mixed and tested to get a proper calibration. This involves the inconvenience of the loss of use of the gauges during the time they are being calibrated, and also the inconvenience of having to transport the gauges back and forth from the lab.
With the foregoing in mind, it is an object of the present invention to overcome the problems and disadvantages of the prior practices discussed above and to provide an improved system for calibrating gauges in a simpler and more time efficient manner without having to transport the gauges back to the lab.